Our invention relates generally to apparatus for data transfer with record media such as flexible magnetic disks, and particularly to a disk drive operating under the control of a host system. More particularly, our invention deals with improvements in the electronic circuitry of a disk drive for the prevention of the undesirable delivery of index pulses to the host system.
The flexible magnetic disk drive of standard construction is not self-contained; it is a slave unit operating under the control of a master, known as a host system, typically comprising a central processor unit (CPU) and a disk drive controller. Also, as disclosed in Shoji et al. U.S. Pat. Nos. 4,583,133 and Tsuyuguchi et al. 4,658,307, two or more disk drives may be interfaced with one host system for independent operation under its control. Each disk drive reads or writes on the associated magnetic disk only when so dictated by a drive select signal fed from the host system. It is also the common practice in the disk drive art to generate in each disk drive a series of index pulses indicative of the angular position and speed of rotation of the magnetic disk and to deliver the index pulses to the host system only when the drive select signal dictates the execution of reading or writing in that disk drive. The host system relies on the index pulses for controlling the various aspects of data transfer operation in the disk drive.
In each disk drive the index pulses may be generated by optically sensing an index hole formed in the magnetic disk in an eccentric position thereon, as is common with disk drives for use with magnetic disks having standardized diameters of 3, 5.25 or 8 inches. Alternatively, in disk drives for use with 3.5-inch magnetic disks, the index pulses may be generated by magnetoelectrically or photoelectrically detecting a permanent magnet or a piece of reflective sheet affixed to a part rotatable with the magnetic disk, for example, the outer rotor of the disk drive motor. Reference may be had to Castrodale et al. U.S. Pat. No. 4,089,029 for more details on the photoelectric detection of the index hole in the magnetic disk, and to Shoji et al. U.S. Pat. No. 4,629,942 for more details on the magnetoelectric or photoelectric detection of index marks on the parts rotatable with the disk.
As far as we are aware, there has been a problem left unsolved in connection with the delivery of the index pulses from the disk drive to the host system in response to the drive select signal. Let us assume that the drive select signal as supplied from the host system is normally high and goes low to command the executive of data transfer in the disk drive. The problem arises as the drive select signal may go low at any moment, with no time relation whatever to the index pulses. Admittedly, even in the prior art data transfer system, no inconveniences have been encountered in cases where the drive select signal goes low during the time intervals between the successive index pulses. The problem does, however, occur when the drive select signal goes low during the duration of any index pulse, as discussed in further detail hereafter.
The known configuration of the disk drive circuitry has been such that the index pulses and the drive select signal are fed more or less directly to a logic gate. When the drive select signal is high, not commanding the operation of the disk drive, the gate inhibits the delivery of the index pulses to the host system. Suppose that the drive select signal goes low during the duration of an index pulse. This index pulse will have only its leading edge portion cut off and will be permitted to go into the host system in the form of a reduced duration pulse. The leading edge of this reduced duration pulse does not correctly represents the angular position of the magnetic disk. This is undesirable because the host system relies on the leading edges of the incoming index pulses for disk formatting and other disk control functions. We will later discuss the problem in more detail in the course of the disclosure of a preferred embodiment of our invention.